1. Field of the Invention
This invention relates to a method for preparing .alpha.-L-aspartyl-L-phenylalanine (hereinafter, referred to as ".alpha.-AP" in abbreviation).
2. Discussion of the Background
.alpha.-AP can be used as a starting material for synthesizing .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter, referred to as ".alpha.-APM" in abbreviation). .alpha.-APM is a peptide sweetener of commercial value with a sweetness approximately 200 times that of sucrose. It has a favorable sweet flavor and is low in caloric value, which contributes to its significant applications in recent years as a dietary sweetener and the demand towards .alpha.-APM has been increased year by year.
Various methods are known for preparing .alpha.-APM. For example, Japanese Examined Patent Publication No. 40069/1976 discloses a method of condensing a strong acid salt of aspartic anhydride and L-phenylalanine methyl ester (hereinafter, referred to as "PM" in abbreviation).
Japanese Examined Patent Publication No. 033479/1989 (corresponding to European Patent No. 58063) discloses condensation of N-formyl-L-aspartic anhydride and PM and subsequently deformylation according to a well-known technique to crystallize .alpha.-APM as the hydrochloride.
A method disclosed in Japanese Examined Patent Publication No. 026133/1980 is directed to condensation of L-phenylalanine (hereinafter, simply referred to as "Phe" in abbreviation) and N-formyl-L-aspartic anhydride in glacial acetic acid, from which formyl groups are removed. .alpha.-APM is crystallized in the presence of methanol and hydrochloric acid for esterification.
A method of obtaining .alpha.-APM hydrochloride (hereinafter, referred to as ".alpha.-APM.multidot.HCl" in abbreviation) from .alpha.-L-aspartyl-L-phenylalanine dimethyl ester and .alpha.-L-aspartyl-L-phenylalanine-.beta.-methyl ester is disclosed in Japanese Patent Laid Open Nos. 219258/1984 and 225132/1984.
In another method, 3-benzyl-6-carboxymethyl-2,5-diketopiperazine is subjected to partial hydrolysis in the presence of water, methanol and strong acid as disclosed in Japanese Patent Laid Open No. 174799/1985 (corresponding to U.S. Pat. No. 4,634,790).
All of these methods are based on a technique to first crystallize and isolate .alpha.-APM as a slightly soluble hydrochloride. The resultant APM.multidot.HCl crystals are neutral-crystallized to obtain .alpha.-APM. The mother liquor separated after hydrochloride crystallization generally contains a large amount of .alpha.-AP derivatives. In addition, the mother liquor often contains a large amount of .beta.-aspartyl phenylalanine methyl ester, .beta.-aspartyl phenylalanine, .beta.-aspartyl phenylalanine-.beta.-methyl ester and .beta.-aspartyl phenylalanine dimethyl ester (hereinafter, referred to as ".beta.-AP derivatives" in general) and other impurities. Accordingly, the conventional methods require either to decompose the derivatives into Phe and aspartic acid (hereinafter, referred to as "Asp" in abbreviation) before recovery of Phe and Asp for recycling or to neutralize, with a base, the hydrochloric acid contained in the liquor in a significant amount before treatment with active sludge or the like.
.alpha.-AP derivatives such as .alpha.-APM can be recovered from the mother liquor crystallized .alpha.-APM.multidot.HCl by means of, for example, neutralizing the mother liquor with a large amount of base (for example, sodium hydroxide and sodium carbonate) to crystallize .alpha.-AP or ester derivatives thereof. In this case, the mother liquor crystallized .alpha.APM.multidot.HCl usually contains hydrochloric acid in a high concentration of 2 through 5 Normal, which yields a large amount of inorganic chlorides as the by-product of the neutralization. In addition, the ratio of desired .alpha.-AP derivatives to undesired .beta.-AP derivatives in the mother liquor tends to be at least 1:1 or less, so that it is impossible to selectively crystallize a desired .alpha.-AP or ester derivatives thereof.
Alternatively, there is a method using an ion exchange resin to obtain .alpha.-AP derivatives. This method requires, however, a great amount of ion exchange resin and large volume of liquid waste is discharged to obtain .alpha.-AP derivatives in unsatisfactory yields. With any one of these methods, it is extremely hard for efficiently and industrially obtaining .alpha.-AP derivatives from the mother liquor crystallized .alpha.-APM.multidot.HCl.
Effective recovery of .alpha.-AP from mother liquor crystallized .alpha.-APM.multidot.HCl thus brings major challenges in an .alpha.-APM preparation process, which various conventional methods have never been achieved.